Na ion ionophores involved in action potential generation in electrically excitable cells have been found to have three separate receptor sites for toxins. The alkaloid toxin aconitine, batrachotoxin, grayanotoxin and veratridine interact specifically with one receptor site and cause persistent activation of action potential Na ion ionophores. Scorpion toxin binds specifically to a second receptor site and interacts cooperatively with the alkaloid toxins. Saxitoxin binds to a third receptor site and inhibits the Na ionophore. In this research, the properties and the regulation of destiny of action potential Na ion ionophores will be investigated using these specific toxins as experimental tools and cultured excitable cells including cultures of synchronously beating heart cell aggregates as an experimental system. The properties, stoichiometry, and allosteric interaction among these three receptor sites will be studied in intact cells using measurement of ion flux and specific binding of (125I) scorpion toxin and (3H) saxitoxin. Receptor sites will be solubilized and identified using (3H) saxitoxin and a photoactivated derivative of (125I) scorpion toxin. The properties and physical relationship of these receptor sites will be studied in solubilized preparations. The goal of these experiments will be to understand the relationship between the allosteric interactions among these receptor sites and the processes of activation and inactivation of the Na ion ionophore. The effects of antiarrhythmic drugs on the allosteric interactions among the receptor sites for specific toxins will be studied. The goal of these studies will be to elucidate feedback control mechanisms by which the density of Na ion ionophores, and therefore the electrical excitability, of heart cells might be regulated. Impairment of such regulatory mechanisms may play an important role in the etiology of chronic arrhythmias and other aspects of heart disease.